Sound recording



Aug. 8, 1933. K. F. MORGAN SOUND RECORDING Filed Oct. 1, 1950 FIG. 2

INVENTOR K. E MORGAN A TTORNEV Patented Aug. 8, 1933 UNITED STATESPATENT OFFICE SOUND RECORDING Application October 1, 1930, Serial No.485,648,

and in Great Britain September 4, 1930 10mi s.

This invention relates to sound recording and particularly to a methodof recording by photographic means a wider range of sound volumes thancan be recorded by known methods.

5 In any method of recording sound the range of the volumes of soundwhich can be satisfactorily recorded is limited by the properties of therecording medium used and the handling which the completed recordreceives. The least volume which may be recorded is the volume which inreproduction can be satisfactorily distinguished above the noiseproduced by irregularities in the structure of the mediums used inrecording and reproducing and by the dirt} l6 scratches, etc. whichcollect on the record during its useful life.

The object of this invention is a system of sound recording which willproduce a reduction of the volume of noise produced from a photographicsound record during the reproduction of the sound from the record. Thisreduction in the volume of the noise produced permits smaller volumes ofsound to be satisfactorilyrecorded and thus increases the range ofvolumes of sound which may be recorded. Or this invention will provide alarger ratio between the present minimum volume of sound and the volumeof noise produced during reproduction.

Experiments have shown that the volume of noise produced during thereproduction of a photographic sound record is approximatelyproportional to the average transmission of the positive sound record.The average transmission of a photographic sound record is considered tobe equal to the transmission of the unmodulated portions of the soundrecord. The transmission of a photographic medium is defined as theratio of the light transmitted through the medium divided by the lightincident on the medium and 40 for convenience is commonly expressed as apercentage.

A feature of this invention is a method of producing a positivephotographic sound record in which the average transmission of the sound45 record varies with the volume of the sound recorded. Thus for smallvolumes of sound a dense record is produced and the noise producedduring reproduction is a minimum.

In the reproduction of sound from a photographic sound record it hasbeen shown that the volume of the reproduced sound is proportional tothe product of the average transmission of the sound record multipliedby the percentage variar tion in this transmission due to the modulationof the transmission in accordance with the sound. In order to preventdistortion or the volume of the sound, when the transmission of thepositive sound record is reduced, the percentage modulation of thistransmission must be inversely increased. For convenience in operation,the percentage modulation of the transmission is made as nearly 100% asmay be practical, as this will give the smallest values of transmissionof the positive sound records. Other values of modulation may be used ifdesired.

A photographic positive is an inverse copy of the photographic negativefrom which the positive was printed thus if the positive sound recordhas a small average transmission the negative sound record must have acorrespondingly large. transmission. The exposure to which aphotographic material is exposed is defined as proportional to theproduct of the intensity of the light incident on the.materialmultiplied by the time during which the material is subjected to thislight. I If the complete photographic process is assumed to becontrolled so that no distortion is produced, then the transmission ofthe positive sound record will be proportional to 30 the exposure of thenegative. Accordingly, to produce a positive sound record having a smallaverage transmission, the exposure of the negative must becorrespondingly small. In accord-. ance with this invention the exposureis reduced 35 to produce a negative sound record having a large averagetransmission.

A further feature of this invention is a method of recording sound byphotographic means in which the exposure of the negative sound recordvaries directly with volume of the sound recorded and the light whichperforms the recording is virtually completely modulated for all volumesof sound.

In a preferred embodiment of this invention, a light valve of the typedisclosed in U. 8. Patent 1,638,555 granted August 9, 1927 to E. C.Wente is employed. In using a light valve of this type for recordingsound by the method disclosed in the above patent the intensity of therecording light does not change but the time of exposure is varied asthe height of the slot defined by the ribbons is varied by the movementof the ribbons. The average time of exposure will be controlled by theaverage spacing of the ribbons or, in other words, by the height of theslot formed when the ribbons are not displaced by a current flowing inthem.

In accordance with the invention the sound waves are detected as by amicrophone and the 110 resulting electric currents amplified in theknown way and applied to the ribbons through a large condenser. Aportion of the: amplified sound current is tapped off the output of theamplifier and fed to a controlling device, for example, a detector. Theoutput of the detector will be a pulsating unidirectional current Whosemagnitude varies in accordance with the volume of the sound. The ribbonsof the light valve are connected in series with the output circuit of anamplifying vacuum tube. The grid bias of the vacuum tube is adjusted sothat the ouput current of the tube is of such magnitude that the ribbonsare nearly touching and the height of the slot defined by the ribbons issmall. The

output of the detector is applied to the grid of.

the amplifying vacuum tube in such manner that the output current of thevacuum tube varies inversely with the output of the detector. The heightof the slot defined by the ribbons is thus enlarged in direct proportionto the volume of the sound being recorded. At the same time the ribbonsare vibrated by the amplified currents from the sound detector and varythe time of exposure of the film in accordance with the modulationproduced by the sound. As the amplitude of the vibrations is controlledby the volume of the sound, and as the height of the slot issimultaneously controlled by detected currents varying with the volumeof the sound it will be apparent that the slot is always just largeenough to allow the vibration. In other words, the light passing throughthe valve is virtually completely modulated for all volumes of sound.

It is well known that the definition of a photographic sound record isimproved when the recording slot is made very small, and, due to theinfinite height of the recording and reproducing slots, that there is aloss of definition and power in the reproduction of the higherfrequencies of sound. It has been found experimentally that speech andmusic sounds of low frequency contain greater power or volume than thoseof high frequency and when recorded the sounds of low frequency willopen the ribbons of the light valve to normal spacing producing a recordhaving as good definition as the records produced by previously knownmethods. The high frequency sound being of low volume, the ribbons ofthe light valve will be nearly closed, and the definition of the recordof the high frequency sound will be better than the records producedheretofore and will partially offset the loss due to the finite heightof the recording slot. In the drawing Fig. 1 represents in schematicform an embodiment of the invention;

Fig. 2 represents diagrammatically a film sound recording system.

In Fig. 1 sound waves are detected by a known type of microphone 10 andcause the current from the battery 11 to be modulated in accordance withsound. The sound modulated currents in the primary winding oftransformer 12 induce similar currents in the secondary winding of thetransformer. The output of transformer 12 is amplified in a known typeof amplifier 13. Part of the output of the amplifier 13 is appliedthrough a transformer 14 to the movable elements 15 of a light valvewhich for convenience is shown as a modification of the light valvedisclosed in U. S. Patent 1,638,555 August 9, 1927 to E. C. Wente. Themodulated speech currents cause the movable elements of the light valveto vibrate in accordance with the current.

In Fig. 2 the image of the filament of a. suitable lamp 16 is focused bythe lens 1'7 on the small slot formed by the movable elements 15 in theorifice 18 pierced in the pole pieces of a permanent magnet 19. Suitablepole faces concentrate the magnetic field of the magnet 19 to form auniform magnetic field in which the movable elements 15 are immersed. Anelectric current flowing in the movable elements 15 will cause theelements to be displaced by the reaction of the magnetic fields due tothe current and the magnet. The image of the slot is focused by thelenses 20 and 21 on a photographic film 22 moving at uniform speed pastthe point of exposure. An aperture plate 23 holds the film smooth at thepoint of exposure and prevents diffused light from fogging the record.

In Fig. 1 a portion of the sound modulated currents from the amplifier13 is applied through a tapped transformer 24 to the grid of a vacuumtube 25. The bias applied to the grid of the vacuum tube 25 may bevaried by means of the potentiometer 26. The sound modulated input tothe grid'of the vacuum tube 25 may be varied by means of the tapsbrought out from the secondary of the transformer 24. The plate currentof the vacuum tube may thus be adjusted to any convenient value and thesound modulated input adjusted so that there is obtained the highestcoefiicient of detection. Plate current is supplied to the vacuum tube25 from a battery 27 through the coupling resistances 28 and 29 whichmay conveniently be in the form of a tapped resistance. The combinedresistances 28 and 29 are of suitable value to assist in producingcomplete detector action in the vacuum tube 25.

The detected currents in the output of the vacuum tube 25 produce apulsating voltage across the resistance 29 which is applied to thegridfilament circuit of a vacuum tube 30. A steady 115 bias is appliedto the grid of the vacuum tube 30 by a battery 32 and the value of thisbias may be adjusted by the potentiometer 31 shunted across the battery32. Plate current is supplied to the vacuum tube 30 from .a battery 33and 120 flows from plate to filament in the tube, along wire 34, wire 35and movable elements 15 of the light valve wires 36 and 3'? to thebattery. A condenser 38 of large capacity prevents this steady currentfrom flowing in the winding of 25 the transformer 14, yet permits thesound modulated currents to be applied to the movable elements 15 of thelight valve. By adjusting the bias on the grid of the vacuum tube 30 bymeans of the potentiometer 31 the plate current flowing 13 through themovable elements 15 may be adjusted so that when no sound modulatedcurrents are applied the slot in the light valve is small. Thisadjustment is made easier by adjusting the variable resistance 39 whichmay be in series with 135 the light valve. The filament heating supplyfor the vacuum tubes 25 and 30 is conventionally represented by thebatteries 40 and 41.

The operation of the circuit shown in Fig. 1 is as follows: When nosound modulated current is 140 flowing the plate current of the vacuumtube 30 will cause the movable elements 15 of the light valve topractically close the slot. The time of exposure of the film is veryshort and a very light negative film will be produced. When 1455 soundmodulated current is applied to the vacuum tube 25 the grid bias isreduced and the plate current increases. The increased plate currentflowing in the coupling resistance 29 increases the negative biasapplied to the grid of 15 the vacuum tube 30 and causes the platecurrent to decrease and permit the movable elements 15 of the lightvalve to open the slot. As the decrease in the plate current iscontrolled by detected sound modulated currents the slot will beenlarged in proportion to the amplitude of the sound modulated currents.By suitable adjustment of the input, the amount which the slot enlargesmay be made equal to the amplitude of vibration of the movable elementsso that the light reaching the film is always virtually completelymodulated. Complete modulation of the lightallows the production of anegative having the maximum transmission but it will be obvious that theopening of the slot may be adjusted so that the amplitude of vibrationis any desired fraction of theopening. The maximum opening will bereached when the plate current of the tube 30 is reduced to zero andwill be limited by the construction and mechanical adjustment of thelight valve.

A choke coil 42 in the output circuit of the detector 25, in combinationwith a condenser 43 between the plate and filament of the detector 25forms a smoothing circuit which aids in cansing the voltage variationsdeveloped across the resistance 29 to follow the envelope of the 311T.plitude of the sound modulated currents and not the instantaneousvariations. A second condenser 44 connected between the plate andfilament of the vacuum tube 30 assists in the smoothing action and aidsin causing the ribbons to open promptly when a sudden loud sound is tobe recorded.

While the invention has been applied to a specific form of light valveit will be obvious to those skilled in the art that the invention is notlimited to the specific type of light valve disclosed.

What is claimed is: l. The method of recording sound which comprisesmoving a photographic medium at uniform speed through a beam of light ofconstant invirtually complete modulation of the light passing throughsaid orifice for all volumes of sound recorded.

3. In a system for recording sound on a moving photographic medium, incombination, a source of light of constant intensity, a light valvehaving movable elements defining a slot controlling the transmission oflight from said source to said medium, means for applying electriccurrent varying in accordance with sound to vibrate the movable elementsof said light valve, a source of unidirectional current, a circuit forapplying unidirectional current from said source to said movableelements to cause said elements to tend to decrease the size of saidslot in the direction of motion of the medium, and a controlling deviceactuated in accordance with the envelope of the amplitude variations ofa sound to vary the current from said source.

4. In a system for recording sound on a moving photographic medium, incombination, a source of light of constant intensity, alight valvehaving movable elements mechanically adjusted to define a slotcontrolling the transmission of light from said source to said medium,means for applying a sound modulated electric current to vibrate themovable elements of said light valve, a source of unidirectionalcurrent, a circuit for applying unidirectional current from said sourceto said light valve to cause said elements to tend to decrease the sizeof said slot in the direction of motion of the medium and an impedanceelement in said circuit to vary the current from said source inverselyin accordance with the envelope of the volume variations of a sound.

5. In a system for recording sound on a moving photographic medium, incombination, a source of light, a light valve having movable elementsmechanically adjusted to define a slot of normal height in the directionof motion of the medium controlling the transmission of light from saidsource to said medium, means for applying sound modulated currents tovibrate the movable elements of said light valve, a source of electriccurrent, a circuit for applying current from said source to said lightvalve to cause said movable elements to tend to reduce the normal heightof said slot, an impedance element in said circuit, and controllingmeans energized by sound modulated currents to vary said impedanceelement and cause the height of said slot to increase in accordance withthe amplitude of vibration of said movable elements.

6. The method of recording sound which comprises moving a photographicmedium at uniform speed past an orifice illumined by light of constantintensity, causing the light passing through said orifice to bemodulated to a constant percentage for all volumes of sound, and varyingthe time of exposure of said medium directly in proportion to the volumeof sound recorded.

7. In a system for recording sound on a moving photographic medium, incombination, a source of light, a light valve having movable elementsmechanically adjusted to define a slot of normal height in the directionof motion of the medium controlling the transmission of light from saidsource to said medium, means for supplying sound modulating currents tosaid light valve to vibrate

